Laravel 5 Error installing wapnen/google-cloud-vision-php

I'm using Laravel 5.3, I want to install wapnen/google-cloud-vision-php, when i add composer require wapnen/google-cloud-vision-php, it come out with the error Could not find package wapnen/google-cloud-vision-php at any version for your minimum-stability (stable). Check the package spelling or your minimum-stability. What do that mean?

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